1. Field of the Invention
The present invention relates to a solid-state image sensing device and an image sensing device including the solid-state image sensing device.
2. Description of the Prior Art
In recent years, as an alternative image sensor of a CCD (Charge Coupled Device) type image sensor, a MOS type image sensor has been a subject of interest. This is because the MOS type image sensor has many advantages such as that steady supply of MOS type image transistor is possible because the MOS type image sensor can be fabricated by the same CMOS process as that used for CPU (Central Processing Unit) and memories, and thus existing equipment can be utilized; a drive circuit and a signal processing circuit can be more simply structured compared to the CCD type image sensor; and power consumption can be reduced.
A general MOS type image sensor includes: a pixel array having pixels arranged in rows and columns and vertical signal lines 101 (see FIG. 7A) each of which is provided for each column and transfers a voltage signal photoelectrically converted by each pixel; and a column amplification section having column amplifiers each of which is provided for each vertical signal line 101 and amplifies the voltage signal transferred via the vertical signal line 101. The voltage signal amplified in the column amplification section is passed through a horizontal signal line controlled by a horizontal scan circuit and transmitted via a multiplexer or the like to an output circuit.
FIG. 7A and FIG. 7B are circuit diagrams illustrating an exemplary structure of the column amplification section in a conventional solid-state image sensing device, wherein one amplifier connected to a vertical signal line 101 is shown in FIG. 7A, and amplifiers are shown in FIG. 7B.
The conventional column amplification section includes amplifiers each of which has an amplification transistor M1 and a current source M2. The source of the amplification transistor M1 is connected to ground, and a voltage signal Vin is input to a gate electrode of the amplification transistor M1. One end of the current source M2 is connected to a power supply voltage feed section VDDA, and the other end of the current source M2 is connected to the drain of the amplification transistor M1. The current source M2 is formed of a p-channel type MOS transistor which is operated in, for example, a saturation region. In FIG. 7A and FIG. 7B, a parasitic resistance produced between the power supply voltage feed section VDDA and the current source M2 is indicated by ra, and a parasitic resistance produced between the ground (AGND in FIG. 7A and FIG. 7B) and the amplification transistor M1 is indicated by rg.
In the column amplification section of such a structure, the voltage signal Vin input via the vertical signal line 101 is amplified in the amplification transistor M1, and an amplified voltage signal Vout is taken out at the drain side of the amplification transistor M1. Since one amplifier is provided for each column in the conventional column amplification section, a simply structured source-grounded amplifier is used to suppress an increase in circuit area.
Meanwhile, the threshold voltage Vt (that is, a voltage of the voltage signal Vin) of the amplification transistor is determined on a basis of a power supply potential and a ground potential. Therefore, if the signal voltage Vin is too large when a signal level Vps (that is, a voltage of the Vout) is read out, the value Ids of a current flowing into the current source M2 changes, which changes the power supply potential and the ground potential, leading to a change in threshold voltage Vt. Consequently, a potential of the output Vout of the amplifier also changes, which appears as if a black level at a time when a high intensity signal is input were different form a black level at a time when a dark signal is input.
Amplifiers provided in rows share impedances of the power supply and the ground with each other, the number of amplifiers provided in rows being the same as the number of columns. Therefore, if the value Ids of a current flowing into any one of the amplifiers from the power supply voltage feed section changes, the change may influence on all of the amplifiers. For example, when a high intensity light is input in a part of the pixel array, a current flowing into an amplifier changes, the amplifier reading out the voltage signal from a pixel in the part in which the light is input. Consequently, the change in the current influences on all of the amplifiers, which result in that black levels at the right and left of the high intensity section differ from black levels at the top and bottom of the high intensity section in an output image. That is, a white streak or a black streak appears on the left and right of the high intensity section. This phenomenon is generally called streaking.
To cope with this problem, a technique is disclosed in Japanese Laid-Open Patent Publication No. 2005-252529 in which a limiter is provided at an output of an amplifier provided for each column to limit an output voltage Vout of the amplifier within such a range that a current Ids flowing into the amplifier is stabilized.